An arrangement for the pre-concentration of a sample substance for spectroscopical purposes in the flow injection analysis is known from a paper by Olsen et al in the journal "The Analyst", vol 108, 905-917. Into parallel peristaltic conduits, a peristaltic pump feeds water, a buffer liquid in the form of ammonium acetate and an eluting liquid in the form of nitric acid. An injection valve is arranged in the hose conduit, into which water is fed. The injection valve contains a through-passage and a sample loop, which is adapted to be optionally switched into the flow in the hose conduit. Then the through-passage is switched into the hose conduit, a flow of sample liquid is passed through the sample loop, such that the sample loop is filled with sample liquid. After the injection valve has been changed over, the sample loop filled with sample liquid is connected to the hose conduit carrying the water flow, such that the sample liquid is taken along by the water flow. The water and the sample liquid, respectively, are mixed with the buffer liquid and, in a first valve position of a valve arrangement, flow through an ion-exchanger column from a first end of the ion-exchanger column to a second end. The second end communicates with a waste outlet. In the first valve position of the valve arrangement the eluting fluid flows to a nebulizer and is sprayed into the flame of an atomic absorption spectrometer. Now pre-concentration of the sample takes place in the ion-exchanger column. The valve arrangement is subsequently changed over to a second valve position. In this second valve position, the water and ammonium acetate hose conduits communicate with the waste outlet. The second end of the ion-exchanger column communicates with the hose conduit carrying the eluting liquid. The first end of the ion-exchanger column communicates with the nebulizer of the atomic absorption spectrometer. The eluting liquid flows through the ion-exchanger column in a direction opposite to the previous direction and elutes the pre-concentrated elements to be determined in to the nebulizer and thus into the burner of the atomic absorption spectrometer.
From a paper by Hartenstein et al in "Analytical Chemistry" 57 (1985), 21,25 and a paper by Zhaolun Fang et al in "Analytica Chimica Acta" 200 (1987), 35-49, an arrangement is known wherein a first sample liquid with an associated buffer liquid and a second sample liquid with an associated buffer liquid are fed by a first peristaltic pump. The sample liquids are mixed with the associated buffer liquids in tube coils, which are connected downstream to the peristaltic pump. The thus obtained sample and buffer liquids are passed to a first valve. In a first valve position of the first valve the sample and buffer liquids are passed to a first end of an associated ion-exchanger column. The other second ends of the ion-exchanger columns each communicate with a waste outlet. Then the two ion-exchanger columns are loaded in a parallel with the sample liquid, the sample is pre-concentrated in the columns. A second peristaltic pump feeds an eluting liquid and water. In a first position of the valve the water is passed to the nebulizer of a plasma burner. In this first position of the valve the eluting liquid communicates with a waste outlet. In the second position the valve passes the eluting liquid to the second end of one of the ion-exchanger columns, the first end of which then communicates with the nebulizer. The ion-exchanger column exposed to the eluting liquid is selected by a change-over valve.
By the use of two ion-exchanger columns, which are loaded in parallel, the analysis time can be approximately halved. Instead of the eluting liquid, water is passed to the nebulizer while the ion-exchanger columns are loaded. The water washes the nebulizer and stabilizes the plasma.
In all known arrangements of the present type the ion-exchanger columns have substantially constant cross section over their entire length. The ion-exchanger columns according to the paper by Zhaolun Fang et al in "Analytica Chimica Act" 200 (2987), 35-49 have conically tapered ends. This is to ensure uniform flow. However, the essential portion of the ion-exchanger column, in which the sample is pre-concentrated, is cylindrical.
In order to obtain a high pre-concentration of the sample in the ion-exchanger column, a relatively long pre-concentration time is required with the known arrangements. This results in dead times of the spectrometer and to inadmissibly high consumption of inert gas when a plasma burner is used. Reduction of the analysis frequency results in difficulties with the calibration in routine applications. Increased drifts of the experimental conditions occur namely due to the increase time of the experiment. The drifts have to be taken into account by more frequency calibration actions. This, however, further reduces the efficiency of the method. A further problem therein is the dispersion of the eluted sample slug, which counteracts an increase in the degree of sensitivity and thus makes longer pre-concentration times necessary.